Understanding the interaction between tumor and immune system might help improve cancer immunotherapy. However, little is known how tumor associated antigens (TAA) regulate populations of effector and TR cells found in spontaneous responses in cancer. Since most TAAs are self-antigens it is not known how efficient they are to stimulate effector T cells and induce their clonal expansion. It is also not understood how TAAs induce and sustain regulatory CD4+ T cells (TR) expressing a transcription factor Foxp3+ which have been identified as the major obstacle to effective antitumor immunotherapy. We and others have determined that the population of TR cells in humans and mice is heterogeneous and TR subsets may have different, non-overlapping functions. The majority of TR cells present in healthy mice maintained a stable suppressor phenotype, expressed high level of Foxp3 and an exclusive set of TCRs not used by naive CD4+ T cells. A small TR subset, utilized TCRs shared with effector T cells and expressed a lower level of Foxp3 but could downregulate Foxp3 and produce inflammatory cytokines. This subset was found to dominate TR population in mice undergoing immune response to conventional antigens. It is not known how both TR subsets are regulated by self antigens in normal tissues, preclinical tumor lesions and in the course of tumor growth and which subset dominates the population of TR cells. In addition, it is not known if TR cells compromise cancer immune response starting at the initial stages of tumor growth or following a period of productive immune response. To reveal and understand cellular mechanisms of generating TR cells in cancer we will study how tolerance is established in peripheral tissues in mice bearing preclinically and clinically defined prostate tumors. We will further investigate how blocking expression of Foxp3 in effector and preexisting TR cells alleviates suppressor function of these cells, slows down growth or eradicates tumor cells and affects concurrent immunotherapy. This will be accomplished by modulating the expression of connexin 43, a molecule identified to control suppressor function of TR cells and expression of Foxp3. Finally, we will investigate how conventional hormonal and radiation therapy, commonly used to treat prostate cancer affect the generation and function of effector and TR cells specific for tumor antigens in mice with normal population of TR cells and mice where population of TR cells is greatly reduced.